The present invention relates to an apparatus and method for making a rare-earth alloy magnetic powder compact and a method of producing a rare-earth magnet.
A rare-earth alloy magnet is made through compaction by pressing a magnetic powder that has been obtained by pulverizing a rare-earth alloy. Currently, two types of rare-earth alloy sintered magnets are widely used in various fields: samarium-cobalt magnets and neodymium-iron-boron magnets. Particularly, neodymium-iron-boron magnets (hereinafter, referred to as xe2x80x9cRxe2x80x94Txe2x80x94B magnetsxe2x80x9d, wherein R denotes a rare-earth element and/or Yttrium, T denotes iron and/or a transition metal element substituting part of iron, and B denotes boron.) have been actively employed in various electronic devices because they exhibit the highest magnetic energy product among various magnets and are relatively inexpensive. As an example of a transition metal included in T, Co may be used.
As the variety of applications of rare-earth alloy magnets expands, there is a demand for production of magnets of various shapes. The production of a high-performance motor, for example, requires a plurality of strong anisotropic magnets having a curved surface. In order to produce such an anisotropic magnet, it is necessary to press a magnetic powder oriented in a magnetic field to make a powder compact having a desired shape. A high-performance rotating machine such as a voice coil motor uses a plurality of thin-plate magnets having a C-shaped or arc-shaped cross section. In order to improve the performance of a rotating machine, merely increasing the magnetization of the magnet is not sufficient. It is necessary to obtain the shape of the magnet and the magnetic field distribution in the vicinity of the magnet surface without distortion.
In the prior art, the pressing surface of a mold pressing member of a compacting apparatus is curved to give a desired curved surface to a powder compact. According to such a conventional technique, the pressing surface is mirror-finished.
However, experiments by the present inventors have revealed that where the pressing direction coincides with the direction of the orientation of the magnetic field, if a mirror-finished curved surface exists in the pressing surface, the orientation of the magnetic powder is disturbed, and optimal magnetic properties are not exhibited. Particularly, when a permanent magnet is made from a compact whose orientation has been disturbed and the permanent magnet is used to produce a motor, a non-negligible level of undesirable reluctance torque or cogging torque of the motor is obserbed. A cogging torque is generated due to changes in reluctance of magnetic circuits in the motor as the rotor rotates. When a change in reluctance occurs, a torque (unintended in the design of the motor) is produced. That torque is usually quite small with respect to the intended torque which the motor produces. However, that torque may be large enough to be disruptive in a number of applications for permanent magnet motors, such as electric power steering and electric suspensions for motor vehicles. In such applications, the cogging torque may be enough to be felt by people in the motor vehicle.
It is an object of this invention to provide a compacting apparatus with a curved surface in which orientation disturbance of resulting compacts is suppressed, and which is suitable for making a rare-earth alloy magnetic powder compact whose particles are oriented in a direction parallel to the direction of the magnetic field.
Another object of the present invention is to provide a method of making a rare-earth alloy magnetic powder compact in which the orientation disturbance is suppressed by using such a compacting apparatus, a method of producing a rare-earth magnet, and a rare-earth magnet.
Still another object of the present invention is to provide a powder pressing die set used in such a compacting apparatus.
A powder compacting apparatus of an embodiment of the present invention includes: a die having a through hole forming a cavity; a first punch and a second punch for pressing a rare-earth alloy magnetic powder filled in the cavity; and magnetic field generation means for applying an orientation magnetic field parallel to a pressing direction through the rare-earth alloy magnetic powder in the cavity, wherein at least one of the first and second punches has a curved pressing surface; and the pressing surface is given a shape such as to suppress a movement of particles of the rare-earth alloy magnetic powder along the pressing surface during a pressing process.
In a preferred embodiment, a pattern is formed on the pressing surface, the pattern including concave portions and/or convex portions extending in a direction generally parallel to a reference plane that is perpendicular to the pressing direction.
In a preferred embodiment, the pressing surface includes a plurality of minute surfaces generally parallel to a reference plane that is perpendicular to the pressing direction, and the plurality of minute surfaces extend in a same direction, and the minute surfaces are separated from adjacent surfaces by a step.
In a preferred embodiment, each of the plurality of minute surfaces has a width of 0.1 mm or less.
In a preferred embodiment, concave portions with a depth of 0.1 mm or less and/or convex portions with a height of 0.1 mm or less are arranged on the pressing surface.
In a preferred embodiment, the pressing surface is not mirror-finished and has a surface roughness Ra equal to or greater than 0.05 xcexcm and less than or equal to 12.5 xcexcm.
In a preferred embodiment, the pressing surface is curved in an arch shape as a whole.
A method of making a rare-earth alloy magnetic powder compact of the present invention includes the step of making a compact of a rare-earth alloy magnetic powder by using any of the above-described powder compacting apparatuses.
In a preferred embodiment, the rare-earth alloy magnetic powder is made from an Fexe2x80x94Rxe2x80x94B (wherein R denotes a rare-earth element and B denotes boron) alloy.
A method of producing a rare-earth magnet of the present invention includes the steps of making a compact of a rare-earth alloy magnetic powder by using any of the above-described powder compacting apparatuses; and making a permanent magnet from the compact.
In a preferred embodiment, the rare-earth alloy magnetic powder is made from an Fexe2x80x94Rxe2x80x94B alloy, wherein R denotes a rare-earth element and B denotes boron.
A powder pressing die set of the present invention includes a punch having a curved pressing surface, wherein the pressing surface is given a shape such as to suppress a movement of powder particles along the pressing surface during a pressing process.
In a preferred embodiment, a pattern is formed on the pressing surface, the pattern including concave portions and/or convex portions generally parallel to a reference plane that is perpendicular to a pressing direction.
In a preferred embodiment, the pressing surface includes a plurality of minute surfaces generally parallel to a reference plane that is perpendicular to a pressing direction, and the plurality of minute surfaces extend in a same direction, and the minute surfaces are separated from adjacent surfaces by a step.
In a preferred embodiment, each of the plurality of minute surfaces has a width of 0.1 mm or less.
In a preferred embodiment, concave portions with a depth of 0.1 mm or less and/or convex portions with a height of 0.1 mm or less are arranged on the pressing surface.
In a preferred embodiment, the pressing surface is not mirror-finished and has a surface roughness Ra between 0.05 xcexcm and 12.5 xcexcm.
In a preferred embodiment, the pressing surface is curved in an arch shape.
A rare-earth magnet of the present invention is a rare-earth magnet wherein a pattern is formed on a surface thereof, the pattern including concave portions and/or convex portions extending in a direction generally parallel to a reference plane that is perpendicular to a pressing direction.
Another rare-earth magnet of the present invention includes a surface including a plurality of minute surfaces generally parallel to a reference plane that is perpendicular to a pressing direction, and the plurality of minute surfaces extend in a same direction, and the minute surfaces are separated from adjacent surfaces by a step.
In a preferred embodiment, each of the plurality of minute surfaces has a width of 0.1 mm or less.
Still another rare-earth magnet of the present invention includes a surface including a plurality of strip-shaped flat surfaces extending in a direction generally parallel to a reference plane that is perpendicular to a pressing direction.